The present invention relates to an improved turbulence generator for the head box of a paper machine. In particular the invention relates to improving the quality of the surface of the flow ducts in the turbulence generator by providing the surfaces of the flow ducts with an appropriate coating.
A turbulence generator according to the invention is intended to be placed, for example, between the distributor and the slice channel of the head box. A turbulence generator is usually employed to create slight turbulence in the pulp flowing onto the wire in order to prevent floc formation, and to smooth pressure and velocity differences in the flow so as to supply an as even flow as possible onto the wire along its whole width.
There is known in the field, for example, a head box as disclosed in Finnish patent No. 49441 according to which pulp flowing onto the wire is guided from the distributor past a rotating perforated roll and through a unit of pipes to the slice channel. The perforated roll is employed in this construction to create turbulence in the pulp and to even out flow differences. This reference already suggests that the pipe unit can be made by welding pipes or plates or by drilling metal or suitable plastics components.
A construction according to Finnish patent No. 63978, for example can be considered as the improvement phase following that disclosed above. In this construction the large quantity of drilled holes is replaced by individual separate cores collected to form a pipe unit of the required size. This construction has become particularly advantageous when the so-called stepped flow ducts disclosed, e.g. in German patent publication No. 19 41 424, have been introduced which aim at improving the turbulence generation in the pulp. The cores disclosed by Finnish patent 63978 are intended to be made of plastics by die casting whereby the stepped flow ducts are also easy to produce. Each core thus comprises only one flow duct and the whole unit of pipes is assembled from a large quantity of cores collected inside a separate body construction.
A third phase in the development of a turbulence generator is a construction disclosed in Finnish patent No. 67106. In this construction the turbulence generator is disposed in the head box between the expansion chamber in the flow duct and the distributor, and between the expansion chamber and the the slice cone. The turbulence generator comprises a perforated plate part made of steel which serves as a load-bearing structural part of the head box. The plastic gap or grate part projects from this perforated plate towards the slice cone and is connected to the perforated plate so as to continue the tubular parts of the gap or grate part as plastic coating of the perforations of the perforated plate and further as coating surface of the side of the perforated plate which is next to the expansion chamber. According to this reference the plastic parts are joined to the perforated plate in connection with the mould casting. The turbulence generator of this publication comprises several parts which form a perforated plate divided in the longitudinal direction and which can be separately disassembled and which are connected to the upper and the lower walls of the head box so as to form a structural entity.
Hitherto the most recent improvement in turbulence generators is disclosed in Finnish patent No. 69330. The turbulence generator of this reference comprises one or several successive perforated plates having conventional flow openings which together with metal pipes constitute several superposed and parallel flow passages. These passages are conical and stepped as also the flow passages in the DE publication No. 19 41 424 mentioned above. Further, the flow passages have been joined by a cast plastic piece to form a unit. According to the reference the mould can serve as a structural supporting part of the head box.
When considering the quality of the pulp suspension flowing onto the wire and the desired economy and the manufacturing technique of a turbulence generator, the prior art devices possess some disadvantages.
The quality of turbulence generators made by drilling only of metal or plastics has been questionable, as all the holes have not been fully parallel or uniform as to the quality of the surface. Also, drilling of all the numerous holes and the surface treatment of the drilled holes, viz. grinding and rounding the edges, takes unreasonably long time and is expensive. Also, the metal pipes used in some applications cause trouble as it is very difficult to get pipes of sufficiently high precision and surface quality and even then they are very expensive. Further, the stepped or conical formation of this kind of pipes increases manufacturing costs.
The third method of manufacturing the pipes either of plastics or by coating a raw metal surface with plastics has not been totally accepted by the users. It has not been possible so far to find a plastics material which would endure all the strain that it is subjected to by the continuous contact with the flowing pulp. The plastic materials available today are to some extent porous and they gradually absorb water which results in swelling of the plastics and the surfaces loose their precision and smoothness. The surface coating can even be detached from the metal which has even more serious consequences. Further, the plastics should endure the great pH changes of the pulp (at least 4 to 8) and the effect of various chemicals such as bleaching wastes, pulp adhesives, retention chemicals, fillers, etc. In addition to the chemical effect, some of these substances also have a physical impact on the plastics surface, i.e. they tend to scratch the surface.
The problems of the devices discussed above are various. In most cases the structure is complex and requires high accuracy in manufacturing. Sometimes the materials do not endure the abrasive impact of the pulp. In all cases the manufacturing costs of the turbulence generator are high.
An apparatus avoiding the disadvantages discussed above should be simple as to its manufacturing technique which should allow manufacturing with adequate precision a construction meeting the most varying requirements. The materials of the apparatus should be both physically and chemically durable and the material constituting the flow surfaces should be already formed in the manufacturing phase, i.e. without a finishing treatment, surfaces of the required form and smoothness.